Non square linear system with two unknowns











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I want to know under what conditions exists at least one solution for the following system
$$Ax + By = C$$
where $A$ is $p times q$ matrix, $B$ is $p times q$ matrix and $C$ is a vector of size $p$. Uniqueness doesn't matter. Thanks.










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    up vote
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    down vote

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    I want to know under what conditions exists at least one solution for the following system
    $$Ax + By = C$$
    where $A$ is $p times q$ matrix, $B$ is $p times q$ matrix and $C$ is a vector of size $p$. Uniqueness doesn't matter. Thanks.










    share|cite|improve this question


























      up vote
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      down vote

      favorite









      up vote
      0
      down vote

      favorite











      I want to know under what conditions exists at least one solution for the following system
      $$Ax + By = C$$
      where $A$ is $p times q$ matrix, $B$ is $p times q$ matrix and $C$ is a vector of size $p$. Uniqueness doesn't matter. Thanks.










      share|cite|improve this question















      I want to know under what conditions exists at least one solution for the following system
      $$Ax + By = C$$
      where $A$ is $p times q$ matrix, $B$ is $p times q$ matrix and $C$ is a vector of size $p$. Uniqueness doesn't matter. Thanks.







      real-analysis linear-algebra systems-of-equations






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      edited Nov 14 at 15:48









      Moo

      5,2633920




      5,2633920










      asked Nov 14 at 14:58









      Gustave

      683211




      683211






















          1 Answer
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          The system has a solution if and only if the system



          $$begin{bmatrix}A& Bend{bmatrix}z=C$$ has a solution. Of course, the solution vector $z$ will have $2q$ entries, and if written as



          $$z=begin{bmatrix}x\yend{bmatrix}$$
          then $z$ is a solution of the second system if and only if $x, y$ are solutions to the original system.





          You can interpret this result by looking at ${Ax|xinmathbb R^p}$ as "the set of all linear combinations of columns of $A$". Then, $By$ is the general element of the set of all linear combinations of columns from $B$, which means that $Ax+By$ can be any linear combination of columns from both $A$ and $B$.






          share|cite|improve this answer























          • Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
            – Gustave
            Nov 14 at 15:04












          • @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
            – 5xum
            Nov 14 at 15:06










          • Thank you a lot sir.
            – Gustave
            Nov 14 at 15:07











          Your Answer





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          1 Answer
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          up vote
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          The system has a solution if and only if the system



          $$begin{bmatrix}A& Bend{bmatrix}z=C$$ has a solution. Of course, the solution vector $z$ will have $2q$ entries, and if written as



          $$z=begin{bmatrix}x\yend{bmatrix}$$
          then $z$ is a solution of the second system if and only if $x, y$ are solutions to the original system.





          You can interpret this result by looking at ${Ax|xinmathbb R^p}$ as "the set of all linear combinations of columns of $A$". Then, $By$ is the general element of the set of all linear combinations of columns from $B$, which means that $Ax+By$ can be any linear combination of columns from both $A$ and $B$.






          share|cite|improve this answer























          • Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
            – Gustave
            Nov 14 at 15:04












          • @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
            – 5xum
            Nov 14 at 15:06










          • Thank you a lot sir.
            – Gustave
            Nov 14 at 15:07















          up vote
          2
          down vote













          The system has a solution if and only if the system



          $$begin{bmatrix}A& Bend{bmatrix}z=C$$ has a solution. Of course, the solution vector $z$ will have $2q$ entries, and if written as



          $$z=begin{bmatrix}x\yend{bmatrix}$$
          then $z$ is a solution of the second system if and only if $x, y$ are solutions to the original system.





          You can interpret this result by looking at ${Ax|xinmathbb R^p}$ as "the set of all linear combinations of columns of $A$". Then, $By$ is the general element of the set of all linear combinations of columns from $B$, which means that $Ax+By$ can be any linear combination of columns from both $A$ and $B$.






          share|cite|improve this answer























          • Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
            – Gustave
            Nov 14 at 15:04












          • @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
            – 5xum
            Nov 14 at 15:06










          • Thank you a lot sir.
            – Gustave
            Nov 14 at 15:07













          up vote
          2
          down vote










          up vote
          2
          down vote









          The system has a solution if and only if the system



          $$begin{bmatrix}A& Bend{bmatrix}z=C$$ has a solution. Of course, the solution vector $z$ will have $2q$ entries, and if written as



          $$z=begin{bmatrix}x\yend{bmatrix}$$
          then $z$ is a solution of the second system if and only if $x, y$ are solutions to the original system.





          You can interpret this result by looking at ${Ax|xinmathbb R^p}$ as "the set of all linear combinations of columns of $A$". Then, $By$ is the general element of the set of all linear combinations of columns from $B$, which means that $Ax+By$ can be any linear combination of columns from both $A$ and $B$.






          share|cite|improve this answer














          The system has a solution if and only if the system



          $$begin{bmatrix}A& Bend{bmatrix}z=C$$ has a solution. Of course, the solution vector $z$ will have $2q$ entries, and if written as



          $$z=begin{bmatrix}x\yend{bmatrix}$$
          then $z$ is a solution of the second system if and only if $x, y$ are solutions to the original system.





          You can interpret this result by looking at ${Ax|xinmathbb R^p}$ as "the set of all linear combinations of columns of $A$". Then, $By$ is the general element of the set of all linear combinations of columns from $B$, which means that $Ax+By$ can be any linear combination of columns from both $A$ and $B$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Nov 14 at 15:04

























          answered Nov 14 at 15:01









          5xum

          88.4k392160




          88.4k392160












          • Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
            – Gustave
            Nov 14 at 15:04












          • @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
            – 5xum
            Nov 14 at 15:06










          • Thank you a lot sir.
            – Gustave
            Nov 14 at 15:07


















          • Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
            – Gustave
            Nov 14 at 15:04












          • @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
            – 5xum
            Nov 14 at 15:06










          • Thank you a lot sir.
            – Gustave
            Nov 14 at 15:07
















          Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
          – Gustave
          Nov 14 at 15:04






          Is there anay rank condition on $A$, $B$ and $C$ to ensure the existence? thanks.
          – Gustave
          Nov 14 at 15:04














          @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
          – 5xum
          Nov 14 at 15:06




          @Gustave Once you have the system $begin{bmatrix} A&Bend{bmatrix}z=C$, you can use the standard theorems from linear algebra to determine when a solution exists. Also, "rank condition" on $C$ makes little sense, since $C$ is a vector, not a matrix.
          – 5xum
          Nov 14 at 15:06












          Thank you a lot sir.
          – Gustave
          Nov 14 at 15:07




          Thank you a lot sir.
          – Gustave
          Nov 14 at 15:07


















           

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